There are many occasions in which the need occurs to deliver a stream or jet of liquid at high pressure for cleaning an object such as an automobile, or an engine. The liquid is usually water, mixed with a detergent appropriate to the situation in a desired ratio. It is convenient to provide containers for water and detergent in liquid form, from which the liquids are drawn by a pump through a mixing arrangement, for delivery to a spray nozzle. The water container is usually provided with a float valve for maintaining its level substantially constant. The detergent container is usually replenished from time to time, since a far smaller volume of detergent is used than of water: The level of detergent liquid is therefore not constant. In the washing procedure the pump is turned on and off: such pumps are subject to damage by cavitation which occurs each time the pump starts without full fluid input. The mixing arrangement is typically an aspirator, and if the pressure heads in the two containers are not the same, the mixture ratio departs from that desired. It sometimes happens that detergent liquid finds its way into the water container, to contaminate the water, during periods when the system is not in use, which is undesirable.